This invention relates generally to the field of data transmission and more specifically relates to a current mode differential driver stabilized by providing a feedback voltage from the center tap of the driver output to the current source.
Increasingly so, today""s communication and data processing electronics use lower and lower voltages and faster and faster transmission rates. Voltage levels have gone from five volts to 1.2 volts and lower; data transmission rates are on the order of several gigahertz. Given any bidirectional interface in which data, whether it be optical or electronic, is transmitted and received at a high frequency, it is desirable that the transmission media have a constant source impedance to absorb electromagnetic reflections and not create additional noise. Given modem electronic circuits and boards, some of the sources of electromagnetic discontinuities that create reflections that may cause excess noise include geometric differences in the transmission media and interfaces, multiple loads, different connectors, crossing split reference planes, vias, etc. In other words, for very large scale integrated (VLSI) data processing circuits, it""s a dangerous world; a myriad of both temporal and spatial events to degrade a low voltage high frequency electronic signal exists.
In a differential driver, the signal of interest is actually carried by a difference in the voltage or current between two signal lines, rather than between a signal line and a reference. Typically, the two signals are complementary meaning that they may be at or near the same magnitude but of different signs. Previous differential drivers were voltage driven requiring a relatively large power supply with the voltage source in series with the resistance load. A current mode differential driver was created to accommodate the lower voltages and requiring a smaller power supply and still be very responsive. In a current mode driver with constant current source, the voltage levels of the signals basically float between a positive and a negative voltage rather than between some voltage and a reference. Such a current mode driver may include a p-channel type semiconductor current source that provides current to flow through the terminating resistors and/or load and into an n-channel type current source. If the p-channel and the n-channel current sources are perfectly matched, the voltage between the terminating resistors or load would be centered between the +/xe2x88x92voltage range. The current sources, however, cannot be perfectly matched and as a result, the output common mode voltage shifts which is problematic if the receiver on the other end of the transmission line is seeking a signal at a certain common mode voltage. Because of the above-mentioned discontinuities, if the common mode voltage shifts too much, errors will result.
There is thus a need in the industry for a stable current mode differential driver. Objects, features, and characteristics of the invention; methods, operation, and functions of the related elements of the structure; combination of parts; and economies of manufacture will become apparent from the following detailed description of the preferred embodiments and accompanying figures, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.
To satisfy the above objects and to provide the industry with a solution to the problems stated above, what is presented herein is a stabilized current mode differential driver, comprising: a pfet current source; a nfet current source; a current steering network connected to the pfet current source and the nfet current source; a center tap terminator connected to the current steering network; and a feedback network connected to the center tap terminator to provide feedback to the pfet current source and nfet current source to maintain a constant average common mode voltage.
The feedback network may comprise a first feedback resistor connected to a gate of the pfet current source; and a second feedback resistor connected to a gate of the nfet current source. The driver may further comprise first pfet current mirror reference connected to the gate of the pfet current source; and a second nfet current mirror reference connected to the gate of the nfet current source; wherein the first feedback resistor is also connected to the first pfet current mirror reference and the second feedback resistor is also connected to the second nfet current mirror reference.
A complementary signal may drive the current steering network. The current steering network may comprise two pfets, the sources of each being connected to a drain of the pfet current source and the gates of each being connected to each of the complementary signal; and two nfets, the sources of each being connected to the drain of the nfet current source and the gates of each being connected to each of the complementary signal and the drains of one nfet being connected to the drain of one pfet.
The current mode differential driver may have a differential output. The output may be connected to a transmission line having an impedance matching the center tap terminator.
The invention is also considered a current mode differential driver, comprising: a first current mirror and a first current source having a first bias voltage provided by the first current mirror, the first current mirror and the first current source connected to a supply voltage; a second current mirror and a second current source having a second bias voltage provided by the second current mirror, the second current mirror and the second current source connected to a ground voltage; a first input to the gates of one leg of an H-bridge differential driver; a second input to the gates of a second leg of the H-bridge differential driver, the first and second input being complementary signals; a center tap terminator comprising a center tap between a first terminating resistor and a second terminating resistor, the center tap terminator between a first complementary output at the center of the first leg and between a second complementary output at the center of the second leg, and a resistor feedback network in which the center tap terminator is connected between a first feedback resistor of the resistor feedback network and a second feedback resistor of the resistor feedback network wherein the first feedback resistor is connected to the bias voltage of first current source and the second feedback resistor is connected to the bias voltage of the second current source to maintain an average common mode voltage at the center tap terminator.
The invention is further a method to stabilize the output of a current mode differential driver, comprising the steps of: providing a first current to the current mode differential driver, providing a second current to the current mode differential driver; providing a center tap terminator at an output of the current mode differential driver; providing a voltage at a center tap of the center tap terminator to adjust the first current and/or the second current so maintain an average common mode voltage at the center tap terminator. To provide a voltage at the center tap to adjust the first current and/or second current further, the center tap terminator may be connected to a first feedback resistor whose other end is connected to a gate of a first current source providing the first current. The center tap terminator is also connected to a second feedback resistor having another end connected to a gate of a second current source providing the second current in an arrangement such that the center tap terminator is between the first and second feedback resistors.
The invention may further be considered a stable H-bridge differential driver, comprising: a means to provide current to an H-bridge differential driver; a means to input a complementary signal to an H-bridge differential driver; a means to direct the current through the H-bridge differential driver based on the complementary signal; a means to maintain an average common mode voltage at a center tap of the H-bridge differential driver substantially constant. The means to provide current may be a pfet having a drain connected to a top of the H-bridge differential driver and an nfet having a drain connected to a bottom of the H-bridge differential driver. The means to direct the current through the H-bridge differential driver may further comprise a leg pfet and a leg nfet wherein a gate of the leg pfet is connected to an input signal and a gate of the leg nfet is connected to the complement of the input signal, and the drain of the leg pfet is connected through a center tap terminator to the drain of the leg nfet, and the source of leg pfet is connected to the drain of the pfet and the source of the leg nfet is connected to the drain of nfet. The means to maintain the average common mode voltage to be substantially constant may further comprise a feedback resistor network connected between the center tap terminator and the means to provide current.